1. Field of the Invention
The present invention pertains to wireless telecommunications, and particularly to transmission of AAL2 cells having differing Quality of Service (QoS) requirements.
2. Related Art and Other Considerations
Asynchronous Transfer Mode (ATM) is now commonly used in communication networks. ATM is a packet-oriented transfer mode which uses asynchronous time division multiplexing techniques. Packets are called cells and have a fixed size.
An ATM cell consists of 53 octets, five of which form a header and forty eight of which constitute a “payload” or information portion of the cell. The header of the ATM cell includes two quantities which are used to identify a connection in an ATM network over which the cell is to travel, particularly the VPI (Virtual Path Identifier) and VCI (Virtual Channel Identifier). In general, the virtual path is a principal path defined between two switching nodes of the network; the virtual channel is one specific connection on the respective principal path.
Between termination points of an ATM network a plurality of nodes are typically situated, such as switching nodes having ports which are connected together by physical transmission paths or links. The switching nodes each typically have several functional parts, a primary of which is a switch core. The switch core essentially functions like a cross-connect between ports of the switch. Paths internal to the switch core are selectively controlled so that particular ports of the switch are connected together to allow a cell ultimately to travel from an ingress side of the switch to an egress side of the switch.
A protocol reference model has been developed for illustrating layering of ATM. The protocol reference model layers include (from lower to higher layers) a physical layer (including both a physical medium sublayer and a transmission convergence sublayer), an ATM layer, and an ATM adaptation layer (AAL), and higher layers. The basic purpose of the AAL layer is to isolate the higher layers from specific characteristics of the ATM layer by mapping the higher-layer protocol data units (PDU) into the information field of the ATM cell and vise versa. There are several differing AAL types or categories, including AAL0, AAL1, AAL2, AAL3/4, and AAL5.
AAL2 is a standard defined by ITU recommendation I.363.2. An AAL2 packet comprises a three octet packet header, as well as a packet payload. The AAL2 packet header includes an eight bit channel identifier (CID), a six bit length indicator (LI), a five bit User-to-User indicator (UUI), and five bits of header error control (HEC). The AAL2 packet payload, which carries user data, can vary from one to forty-five octets. Plural AAL2 packets can be inserted into a standard ATM cell. Thus, ATM Adaptation Layer Type 2, i.e., AAL2, facilitates multiplexing of plural AAL2 user connections on a common AAL2 path, established as a single ATM Virtual Channel Connection (VCC).
In an ATM based telecommunications system where different quality of service classes are supported, some connections are more delay sensitive than others. In order to cater to these differing sensitivities, ATM cells of differing priority must be handled. Such ATM cell handling can include specific traffic management per ATM-VCC with weighted fair queuing, early packet discard, available bit rate (ABR) accommodation, and shaping of outgoing traffic according to a traffic contract, for example. Handling of ATM traffic management is specified in the ITU I.371 Recommendation or in ATM-FORUM Traffic Management Specification 4.0.
The Universal Mobile Telecommunications (UMTS) Terrestrial Radio Access Network (UTRAN) is a third generation radio access network which, in some respects, builds upon the radio access technology known as Global System for Mobile communications (GSM) developed in Europe. The UTRAN covers a geographical area which is divided into cell areas, with each cell area being served by a base station. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by a unique identity, which is broadcast in the cell. The base stations communicate over the air interface (e.g., radio frequencies) with the user equipment units (UEs) within range of the base stations. In the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a radio network controller (RNC). The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks. UTRAN is essentially a wideband code division multiple access (W-CDMA) system.
In the UTRAN network, several types of control and user data connections use AAL2. These differing types of connections have differing quality of service (QoS) requirements, such as differing maximum delay tolerance, for example. However, standardized AAL2 does not provide any means for quality of service differentiation between connections on the same AAL2 path. This means that, when connections of different types share an AAL2 path, that shared path has to be dimensioned according to the quality of service requirements of the connection type having the most stringent such requirement (e.g., most stringent QoS).
In view of the foregoing, in some cases it is necessary to establish separate AAL2 paths for the more delay-sensitive traffic (such as conversational traffic, e.g., speech) and less delay-sensitive traffic (such as data traffic) in order to ensure that the most stringent requirements for the more delay-sensitive traffic can be met. In such cases, the AAL2 connections are typically divided into two quality of service (QoS) categories, and an AAL2 path is established to serve each of the AAL2 connection categories. Even the less delay-sensitive connections (e.g., data traffic connections) have strict maximum delay tolerance, although such tolerance is higher than for the delay-sensitive traffic (e.g., speech traffic).
In an implementation of the separate AAL2 paths as described in the preceding paragraph, one AAL2 path can, depending on the eight bit size of the connection identifier (CID), support a maximum of 248 AAL2 connections (some CID values are reserved for other purposes). When a higher number of connections must be supported between adjacent nodes, multiple AAL2 paths must be established.
Thus, conventional practice when handling AAL2 connections with differing QoS categories is to separate the different services on different AAL2 paths (ATM VCCs) with reserved bandwidth on each AAL2 path. The ITU-T AAL2 standards include basic signaling methods to support such separation of AAL2 connections on different AAL2 path types and to set up these AAL2 paths on ATM VCCs with different ATM traffic contracts. For example, AAL2 cells carrying conversational (e.g., speech) traffic may be assigned to AAL2 paths with a first QoS class; AAL2 cells carrying non-conversational (e.g. data) traffic may be assigned to AAL2 paths with a second QoS class (lower than the first QoS class). AAL2 paths of both QoS classes are carried by the same ATM physical link.
In general, the type of ATM service category appropriate for AAL2 paths is Deterministic Bit Rate (DBR), which in the parlance of the ATM Forum is Constant Bit Rate (CBR). For this service category, ATM link resources are reserved according to the peak cell rate (PCR) of each VCC.
In a case in which several AAL2 paths are needed for QoS separation or connection capacity reasons, it is inefficient to allocate a fixed part of the link capacity to each AAL2 path. While the maximum total traffic over a physical link may be possible to estimate based on the capacity of the served radio interfaces (e.g., in the UTRAN), the relative proportion of conversational versus data traffic is more difficult to predict and is expected to change with time. If each path is dimensioned separately according to its expected maximum traffic intensity, more link capacity must be reserved than if the link were dimensioned according to the sum of the traffic on the AAL2 paths of the both types.
Saito, “Effectiveness of UBR VC Approach in AAL2 Networks and Its Application to IMT-2000”, IEICE Trans. Commun., Vol. E83-B, No. 11, November 2000, pp. 2486-2493, proposes a bandwidth management alternative which performs bandwidth management (using unspecified bit rate (UBR) for each VC) at a VP level (virtual path) rather than at a VC level. However, the Saito proposal has various limitations. For example, it assumes that the VP carries nothing other than AAL2 traffic, and also that all AAL2 connections are of the same QoS class. As an example, the Saito proposal does not take into consideration that other types of ATM VCCs, for example AAL5 connections carrying signaling, or operation and maintenance traffic could be included on the VP. Moreover, Saito does not address how differing QoS requirements for differing AAL2 connections should be handled.
What is needed, therefore, and an object of the present invention, is a technique which facilitates bandwidth efficient quality of service (QoS) separation of AAL2 traffic.